1. Field of the Invention
The present invention relates to a reference current generator circuit for low-voltage applications. More particularly, the present invention relates to a reference current generator circuit applying resistance compensation for increasing phase margins.
2. Description of Related Art
FIG. 1 is a diagram illustrating a conventional reference current generator circuit applying a N-channel metal-oxide-semiconductor field effect transistor (NMOSFET) as a second stage of an operational amplifier. All of components illustrated in the circuit of FIG. 1 are within a chip besides an external resistor R and a ground terminal connected to the external resistor R. A non-inverting input terminal of the operational amplifier OPA receives a precise reference voltage VBG from a bandgap reference circuit, and an inverting input terminal thereof is coupled to a chip pad P. Due to a virtual short circuit of the operational amplifier OPA, a level of the pad P is the same to the reference voltage VBG. Therefore, a precise reference current IR can be generated according to the precise reference voltage VBG and the precise external resistor R.
An external stable capacitor is generally coupled to the chip. Loop stability of the circuit is influenced by the external capacitor and a parasitic capacitor, and capacitance variations of the two capacitors. A NMOS transistor is generally applied to serve as an output stage, and an advantage thereof is that the loop stability is easy to be controlled. Since seeing into the chip from the pad P is a low-impedance point, as long as a metal-oxide-semiconductor (MOS) capacitor C coupling the ground is coupled to an output terminal of the operational amplifier OPA to provide a capacitance compensation, the phase margin then can be effectively controlled. However, when an operation voltage VDD of the reference current generator circuit is excessively low, limited by the structure of the circuit and the fabrication process of the MOSFET, headroom available to the MOSFETs M1 and M2 are insufficient, so that the whole circuit cannot operate normally.
FIG. 2 is a diagram illustrating a conventional reference current generator circuit applying a P-channel metal-oxide-semiconductor field effect transistor (PMOSFET) as a second stage of an operational amplifier. Since the reference current generator circuit of FIG. 2 is lack of one layer of the NMOS transistor, the problem of inadequate headroom is mitigate. However, since gains of two stages are added to the circuit, and the output point thereof has high impedance, if the capacitor coupling the ground is used for phase compensation, a relatively large chip area is required to achieve a good stability.